The present invention relates to an industrial vehicle, such as a forklift, having a body and an axle pivotally supported relative to the body, and more particularly, to an industrial vehicle having a stability control apparatus that restricts pivoting of the axle in accordance with traveling conditions and load conditions.
An industrial vehicle, such as a forklift, has a body and an axle, on which wheels are mounted. It is known that the employment of an axle pivotally supported with respect to the body stabilizes the vehicle when the vehicle is traveling. However, when the forklift turns to change directions, centrifugal force acts on the vehicle and applies a lateral force to the body. In such case, the pivotal axle tilts the body sideward. Thus, the vehicle may become unstable when the forklift turns.
Accordingly, Japanese Unexamined Patent Publication No. 58-211903 describes a mechanism for locking a pivotal rear axle. A turning detector detects the centrifugal force applied to the vehicle and locks the rear axle when the force exceeds a predetermined value. Since the rear axle is locked when the vehicle is turned, the lateral tilting of the body is suppressed. Thus, the forklift is stabilized when changing directions.
Japanese Unexamined Patent Publication No. 58-167215 describes an apparatus for actuating the axle locking mechanism. A forklift has forks to lift and carry loads.
The apparatus detects the weight of the load and the height of the forks. When the load is heavy and lifted to a high position, the vehicle""s center of gravity is raised. Under these conditions, the apparatus actuates the axle locking mechanism and locks the rear axle to maintain the stability of the vehicle.
The present applicant has proposed installing a hydraulic damper between the rear axle and the vehicle body. The damper extends and retracts to permit pivoting of the axle relative to the body. Pivoting of the axle is restricted by locking the damper. Hydraulic oil flows through a hydraulic circuit when the damper extends or retracts. An electromagnetic valve is arranged in the circuit. The excitation and de-excitation of the electromagnetic valve shifts the damper between a locked state and an unlocked state. In this structure, the damper incorporates two functions, one as a shock absorber, when the axle is pivoted, and another as an axle lock, for restricting pivoting of the axle.
However, the rear axle may be locked as one of the vehicle""s rear wheels rides over an irregularity such as a bump when a heavy load is lifted to a high position by the forklift. Under such conditions, the axle would be locked in a raised state. When the elevated rear wheel rides off the bump, the locked and raised rear wheel would remain elevated and away from the road surface. In this state, the vehicle""s center of gravity is displaced toward the front. Furthermore, the body would be supported at three points by the two front wheels and by the other rear wheel. In such state, the stability of the vehicle decreases, which hinders stable driving of the forklift.
In addition, if the axle is unlocked with one of the rear wheels in an elevated state, the elevated wheel falls and impacts the road surface. This may be disconcerting to the operator.
The apparatus of Patent Publication No. 58-167215 includes a block inserted between the body and the rear axle to lock the axle. The block cannot be inserted when the axle is pivoted with respect to the body. Thus, the rear wheel cannot be locked in an elevated state.
However, in the apparatus having a locking damper, the rear axle would be locked by the electromagnetic valve regardless of whether the axle is pivoted if, for example, the carried load is heavy and lifted high. This would decrease the stability of the forklift.
This problem not only occurs when the vehicle""s center of gravity is raised. The present applicant has proposed to lock the axle at an early stage depending on how the yaw rate changes (yaw acceleration) and how much lateral force is applied to the vehicle. The axle is locked when the yaw acceleration becomes large, that is, when the forklift operator begins to turn the steering wheel to start turning the vehicle. Therefore, the axle is locked in a substantially level state before the lateral force increases and starts to pivot the axle relative to the body. However, if the axle is locked with one of the rear wheels raised as the forklift turns, the stability of the vehicle will be reduced.
Furthermore, if one of the rear wheels rides over a bump while the axle is locked, the body is lifted by about twice as much in comparison to when the axle is unlocked. This decreases the stability of the vehicle in the longitudinal direction. Additionally, the damper does not function to absorb shocks when the axle is locked. Thus, the stability of the vehicle is greatly affected by the road surface.
Accordingly, it is a first objective of the present invention to provide a stability control apparatus for industrial vehicles that maintains vehicle stability if the axle is locked while in an irregularly pivoted position relative to the vehicle body. It is a second objective of the present invention to maintain vehicle stability if the axle is locked when the vehicle""s center of gravity is raised with the axle in an irregularly pivoted position relative to the vehicle body. It is a third objective of the present invention to maintain vehicle stability by preventing locking of the axle when one of the associated wheels is raised and by suspending locking of the axle until the associated wheels are all located on level ground. It is a fourth objective of the present invention to reduce the influence the road surface has on the vehicle body when the vehicle is traveling while the axle is locked. It is a fifth objective of the present invention to maintain vehicle stability if the axle is unlocked after having been locked due to the raising of the vehicle""s center of gravity. It is a sixth objective of the present invention to maintain vehicle stability, at least when the vehicle is not moving, while the axle is pivoted relative to the body and while the vehicle""s center of gravity is raised.
To achieve the above objectives, the present invention provides an apparatus for controlling stability in an industrial vehicle. The apparatus includes an axle supported to pivot vertically relative to a body of the vehicle. A restricting mechanism restricts pivoting of the axle. An operating status sensing device senses either the status of the vehicle""s motion when the vehicle turns or the status of a load carried by the vehicle. A pivot angle detector detects the pivot angle of the axle relative to the body. A controller selectively actuates and de-actuates the restricting mechanism based on the state detected by the status sensing device and the pivot angle.
In a further aspect of the present invention, a method for controlling the stability of an industrial vehicle is provided. The vehicle has a pivotal axle and a restriction mechanism. The restriction mechanism is located and connected between the axle and a body of the vehicle. The restriction mechanism locks and releases the pivotal axle. The method includes the step of sensing the operating status of the vehicle. The operating status includes at least one of a sensed load characteristic that is indicative of the position of the vehicle""s center of gravity, the vehicle""s yaw acceleration, and the centrifugal force acting on the vehicle. The method further includes the step of sensing the pivot angle of the axle with respect to a reference plane. The reference plane includes the pivot axis of the axle and is fixed with respect to the body. The method also includes the step of selectively locking and releasing the restriction mechanism depending of the operating status of the vehicle and the pivot angle. The pivotal axle is released when the absolute value of the pivot angle exceeds a predetermined value.